1. Field Of The Invention
This invention relates to a method of and an apparatus for manufacturing semiconductor devices and, in particular, to a method of and apparatus for manufacturing semiconductor devices using copper lead frames with no silver plating.
2. Description Of The Related Art
FIGS. 6 and 7 show a conventional plastic package semiconductor device. The device shown includes a copper alloy lead frame 2 having a die pad 3, inner leads 6, and outer leads 8. The die pad 3 and the inner leads 6 are plated with silver 10. A semiconductor pellet 1 is attached to the silver plating 10 of the die pad 3 by means of a die bonding material 4. The semiconductor pellet 1 has electrodes 5 respectively connected to the inner leads 6 of the lead frame 2 through bonding wires 7 made of gold or the like having a diameter of 25 to 50.mu.m. One end of each wire 7 is ball bonded to one of the electrodes 5 on the semiconductor pellet 1 and the other end of each wire is stitched to the silver plating 10 on the surface of the corresponding lead 6. The semiconductor pellet 1, the die pad 3, and inner leads 6 of the lead frame 2, as well as the wires 7, are encapsulated by a resin 9 with only the outer leads 8 exposed.
The bonding of the wires 7 is performed using a capillary tip 11 as shown in FIG. 8. In the stitch bonding section S in FIG. 7, the wire 7 is pressed against the surface of the inner lead 6 with the tip end of the capillary tip 11 and is thereby deformed. Afterwards, the section is heated, causing mutual diffusion of the metals in the bonding section. FIG. 9 is a plan view showing the stitch bonding section S when the bonding has been completed.
The reason for plating the surfaces of the inner leads 6 of the lead frame 2 with silver 10 is as follows. As shown in FIG. 10, silver is less likely to oxidize than aluminum, the main material of the lead frame 2. Thus, by plating the surfaces of the inner leads 6 with silver 10, stitch bonding can be easily performed while heating the bonding section in the air, there being no fear that the surfaces of the inner leads 6 will be oxidized.
However, with the recent trend toward very dense semiconductor devices, the number of electrodes 5 on the semiconductor pellet 1 has increased. At the same time, the package has become more compact and, as a consequence, the distance between adjacent inner leads 6 has become as small as 0.4 mm or less. Thus, when, under high temperature and humidity, voltage is applied to the semiconductor device in order to operate it, electromigration occurs between the inner leads 6 plated with silver which can lead to an electrical short circuit. Furthermore, silver plating increases the production cost of semiconductor devices since silver is an expensive metal.